Coal pretreater and ash agglomerating coal gasifier

ABSTRACT

Caking coal is converted to fuel gas by treatment in specific pretreatment and gasification zones. Pulverized coal is first pretreated by a mild oxidation in a fluid bed, with heat removal, at a temperature of about 700*-800 *F. to destroy the caking properties of the coal. The resulting gases and the pretreated coal are then passed to a fluid bed gasification zone maintained at the same pressure as the pretreatment zone. The fluid bed is positioned in the lower portion of the gasification zone and converts the coal to a gaseous mixture consisting essentially of CO, H2, CO2, and CH4. However, small amounts of oils and tars are also evolved. The oils and tars in the gaseous mixture, are then thermally cracked in the upper portion of the gasification zone to produce a fuel gas relatively free of oils and tars. A specific feature of the pretreatment zone enables removal of high density inorganic materials.

United States Patent Matthews COAL PRETREATER AND ASH AGGLOMERATING COALGASIFIER [75] Inventor: Charles W. Matthews, Darien, lll. Attorney,Agent, FirmMlinare, Allegretti, Newitt & Witcoff [73] Assignee:Institute of Gas Technology,

Chicago 57 ABSTRACT [22] Filed: Oct. 29, 1973 Cakmg coal is converted tofuel gas by treatment in PP BIO-1410376 specific pretreatment andgasification zones. Pulver-' ized coal is first pretreated by a mildoxidation in a 52 US. Cl. 48/202; 48/203; 48/206; fluid with heatremovah at a tempmwm of 201/9; 201/31 about 700-800 F. to destroy thecaking properties of [51] Int. Cl. Cj 3/16 the coal- The resulting gasesand the pretreated coal [58] Field of Search 48/202, 197 R, 206, 209,are Passed to a fluid bed gasification Zone main- 48/21O 203; 201/9, 31,38 41 tained at the same pressure as the pretreatment zone. The fluidbed is positioned in the lower portion of the [56] References Citedgasification zone and converts the coal to a gaseous mixture consistingessentially of CO, H CO and UNITED STATES PATENTS CH However, smallamounts of oils and tars are also 2,336,466 12/1943 Chatterton et al48/206 X evolved The oils and tars in the gaseous mixture, are 2582-7123 m 53 2? then thermally cracked in the upper portion of the 41 a;48/202 X gasification zone to produce a fuel gas relatively free 307O51512/1962 g; 201/9 X of oils and tars. A specific feature of thepretreatment zone enables removal of high density inorganic materials.

6 Claims, 2 Drawing Figures RAW 6 727 TREAT/N6 COAL FEED LOCK 1 /04 5?PfiE-TflfATAlf/VT F01? B/I'l/M/IV0U5 co/ms H 6 arm/W f :1 GENEQAWO/V i56 AIR 5 7o soups Fit'Dl-R Za 5 A/R STEAM 20 ASH LOCK HOPPER 28 PrimaryExaminerS. Leon Bashore Assistant Examiner--Peter F. Kratz(WATER-F/LLED) COAL PRETREATER AND ASH AGGLOMERATING COAL GASIFIERBACKGROUND OF THE INVENTION This invention relates to the production offuel gas from coal. In particular, this process relates ,to coalgasification wherein the coal is processed through special pretreatmentand gasification zones to prevent agglomeration of the coal duringgasification and to produce a gas product that is relatively free ofoils and tars.

One of the main sources of atmospheric pollutants today is from coalfired electric utility boilers. In these installations, a clean fossilfuel, such as natural gas, is not a practical substitute for coal in thegeneration of electricity because of its scarcity and cost. Furthermore,the available supply of clean fuel may combat pollution more effectivelywhen used to fulfill residential and small commercial needs.

By way of example, the combustion products of coal contribute one-eighthof the total atmospheric pollutants emitted in the United States,including one-half of the sulfur oxides and one-quarter of both thenitrogen oxides and of the particulates. Sulfur emissions from coalcombustion may be reduced by (a) using lowsulfur coal, (b) cleaninghigh-sulfur coal by physical methods, (c) removing sulfur oxides fromcoal combustion gases, ((1) removing sulfur during the combustion step,(e) producing de-ashed lowsulfur fuel by solvent processing of coal, and(f) gasifying coal and removing sulfur from the gas before combustion.

The last method, coal gasification with gas cleaning before combustion,appears to offer the greatest reduction in sulfur emission since most ofthe sulfur in gasified coal appears as hydrogen sulfide. The removal ofthis hydrogen sulfide presents no great problem, however, since severaldifferent commercial gas cleaning processes are available today whichcan reduce the hydrogen sulfide content of gas streams from coalgasification to less than ppm, and some processes can remove hydrogensulfide to 1 ppm or less.

As mentioned, processes are known for the conversion of coal to a fuelgas. In Williams, US. Pat. No. 2,805,189, coal is pretreated in a fluidbed prior to gasification at a temperature below about 600F. Theresultant off gases are not recovered and a special solids transfer lineis provided to insure that these gases do not enter the gasificationreactor. This results in a loss of some volatile hydrocarbons, lowersoverall process yield, and presents an additional gas stream that mustbe purified before disposal.

In Howard, US. Pat. No. 2,582,712, particulate coal is pretreated at 900l400F. in admixture with a large volume of residue from a gasificationreaction zone. Specifically, about 30 units of hot residue from thegasification reactionare admixed with a single unit of fresh feed coalto rapidly heat the fresh feed to a noncaking temperature. This processrequires the circulation of large amounts of residue thereby increasingthe cost of the unit and subjecting the unit to considerable wear due tothe abrasive nature of the residue.

SUMMARY OF THE INVENTION It is an object of this invention to provide aprocess for efficiently converting a caking carbonaceous'mate rial suchas bituminous coal, by gasification, to a fuel gas relatively free ofoils and tars containing hydrogen, carbon monoxide and methane.

It is another object of this invention to effectively pretreat a coalsuch as a caking coal, in a pretreatment zone to provide a noncakingfeed for a gasification reaction zone wherein the pretreatment andgasification zones are in direct fluid communication and are maintainedat the same pressure.

It is still another object of this invention to provide a method forremoving gangue and other inorganic solid material from a coal feedprior to gasification.

The process of the present invention relates to the conversion of afinely divided carbonaceous material containing ash such as bituminouscoal, to a fuel gas. In addition to a caking material such as bituminouscoal, other solid carbonaceous material such as oil shale, tar sands,etc. may be processed according to the present invention. This fuel gas,also known in the art as producer gas or low BTU gas, has a heatingvalue of between 300 BTU per cubic foot and prferably between 250 BTUper cubic feet, and is particularly suited for use in a tired boiler forthe generation of electricity. In the first step of this process, afinely divided carbonaceous material, such as a coal pulverized to asize of about 0 X /1 inches to O X /zinches is pretreated by mildoxidation with limited amounts of air at a temperature of about 700 800F. for a time sufficient to destroy the caking properties of the coal.Preferably the coal is pretreated at this temperature for about 10 to 30minutes. This pretreatment is effected in a fluidized pretreatment zoneand results in the oxidation of the surface of the particles to preventagglomeration in the subsequent gasification step. Since heat is evolvedduring this mild oxidation step, it is necessary to withdraw heat fromthe pretreatment zone to maintain close control on the pretreatmenttemperature and to avoid inadvertant agglomeration or ignition of thecoal particles in the fluidized pretreatment bed. This temperaturecontrol is preferably effected by indirect heat exchange with water toproduce steam. This steam in turn is produced in sufficient quantitiesto supply all the steam required in the subsequent gasification step.

The pretreated carbonaceous material and the resultant gases are passedfrom the upper portion of the pretreatment chamber or zone directly toan adjacent gasification zone which is maintained at the same pressureas the pretreatment zone. The pressure of the gasification zone can bemaintained at atmospheric or superatmospheric pressures depending on thequality of the gas desired. Higher pressures, in general, increase theBTU value of the product gas. The pretreated carbonaceous material isthen maintained as a fluidized bed in the lower portion of thegasification zone by means coarse particulate material having arecoverable carbon value. This gas-solids mixture is separated in twostages to provide a coarse particulate fraction, and a fine particulatemixture. The coarse particle mixture is returned to the fluidized bedfor recovery of the carbon contained therein and the fine particles inturn are passed to the bottom ash agglomerating section of thegasification zone. This agglomeration-section is maintained at atemperature slightly higher than that of the fluidized bed by meansknown to the art to cause these fine materials to agglomerate with theash removed from the fluidized'gasification bed.

In addition, this invention removes shale, gangue, dirt, or rockspresent in the coal prior to the gasification of the coal. If thismaterial is passed to the gasification reaction, it can interfere withthe removal of ash from: the bottom of the fluidized bed and dilute thefluidized bed. Therefore, according to the present invention, thismaterial,.which as a density higher than that of the pulverized coal, iswithdrawn from the lower portion of the fluidized pretreatment zone fordisposal.

Other objects and embodiments will become apparent by reference to thefollowing more detailed descrip tion of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side sectional view of apreferred apparatus used in the present invention and the variousprocess flows embodied therein to convert coal to a fuel gas.

FIG. 2 is a top cross-sectional view of the pretreatment andgasification chambers taken along section .line 2-2 of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION and the off gases containinghydrocarbons and carbon oxides are essentially oxygen free. Thetemperature within pretreatment zone 8 is carefully controlled between700800F., preferably between 750800F., by a cooling water stream 18passing through heat exchange coil 16 positioned within fluidized bed10. The heat generated in the fludized bed by the controlled combustionor oxidation of the coal converts the water to steam thereby efficientlyremoving heat from the pretreatment zone as steam stream 20. This steamstream 20 in turn is then passed to the gasification zone in a manner tobe described in greater detail hereinafter to supply the steam requiredtherein.

A specific feature of pretreatment zone 8 is the ability to removeslate, gangue, rocks, etc., present in the feed coal from the bottom ofthe pretreatment zone to prevent the passage of this material to thegasification zone. More particularly, the pulverized coal particles perse have a density appreciably less than the higher density gangue, rocksetc. As a consequence, as the air entering through line 12 passesthrough the openings in grate 1 1, a selective separation of the lightercoal particles from the more dense gangue particles occurs. Theconcentration of gangue particles occurs immediately above grate 11 fromwhich they may be selectively withdrawn via line 14 for suitabledisposal.

The top portion of fluid bed 10 is allowed flow over and pass throughconduit 9 to gasification chamber or Referring to FIG 1, a pulverizedcaking bituminous --l, rnaintained as a fluidized bed 10 above grate 11by an 'o x y gen containing gas such as air entering via line 12.

The amount of gas or air which enters pre-treatment zone 8 and theoxygen content of the gas are adjusted to oxidize about 10% of thecarbon value of the coal passed to pretreatment zone 8. This oxidationserves to merely oxidize the surface of the coal to prevent the coalfrom agglomerating when it is passed to the higher temperaturegasification zone 30. Typically, about 1 to about 1.5 SCF of oxygenminimum (as an air stream) are used per pound of coal passed to thepretreatment zone. As a consequence, the overall conditions withinpretreatment zone 8 result in a deficiency in oxygen zone 30. It shouldbe noted that gasification zone 30 is in direct fluid communication withfluid bed 10. As a consequence, all of the gases generated in fluid bed10 are passed directly to gasification zone 30, and the pretreatmentzone and gasification zone are maintained at the same pressure. The coalpassed to gasification zone 30 is maintained as a fluid bed 32positioned in the gasification section 33 of zone 30 by air and streamentering the bottom portion of the fluidized bed through conduits 27 and29 in a manner known to the art which will be described in furtherdetail hereinafter.

A single fluid bed 32 is illustrated in FIG. 1. However, for productionof higher heating value gas more than one fluidized bed may be used. Inthis embodiment, the pretreated coal would flow into an upper fluidizedgasification bed which reacts with gases from a lower gasification bed.Unreacted char from the upper bed in turn would flow to the lowergasification bed. The coal in fluid bed 32 is converted by a knownreaction with the air and steam to a mixture of hydrogen, carbonmonoxide, methane, nitrogen and small amounts of oils and tars. Themajority (i.e., -70%) of the steam required in the gasification reactoris withdrawn from the steam in line 20 after intermediate processing,not shown, as generated in pretreatment zone 10, via line 24 and isadmixed with air entering via line 26. The resultant air steam mixtureis passed to the lower portion of gasification zone 30 via line 27, intoa concentric solids free space 35 and passes upwardly through theopenings in grate 34 to maintain fluid bed 32 in constant suspension,motion and circulation. Either make up steam or excess steam whenproduced can be added or removed respectively via line 70. Of course,all of the steam required in the gasification zone can be added from anindependent source via line 70.

Fluid bed 32 proper is maintained at a temperature below theagglomeration temperature of the ash in the coal and preferably ismaintained at a temperature above l,500 and preferably about 1,7001,900F. This exact temperature is a function'of. a specific coalprocessed in the gasification zone and is readily determined withoutundue experimentation by those trained in the art.

The bottom section of gasification zone 30, however, is maintained at atemperature above the agglomeration temperature of the ash and comprisesan inverted conical shaped grate 34 sloped downwardly toward Venturinozzle 36 in the manner illustrated in .lequier,

et al., US. Pat. No. 2,906,608, the teachings of which of an-air steammixture entering via line 29 and is in the range in whichv the coal ashbecomes sticky. At these high temperatures (i.e. 50 200F. higher thanthe bed proper) the ash particles stick together and agglomerates areformed which can be withdrawn when they reach a predetermined size viapipe 40 and passed to lock hopper 42. A portion of the steamproduced inpretreater 8 is removed from line 20 via line 22 and admixed with airentering via line 28 and the resultant air steam mixture passed via line29 to a point below constriction 37 in venturi nozzle 36 to create thehigh temperatures and separation desired. As described in greater detailin Jequier, et al., this air steam stream in constriction 37 is ofsufficient velocity (such a greater than 40 ft/sec and preferably about60 to 100 ft/sec) to prevent fine coal particles from flowing throughconstriction 37. The ash particles, however, have a somewhat higherdensity than the coal particles, and although these ash particles do notinitially have sufficient mass to fall downward through the gas flowingin venturi 36, at the higher temperatures present within the throat ofthis nozzle the individual ash particles agglomerate together until theyform a particle having sufficient mass to pass down through constriction37.

As indicated, the hot agglomerated ash particles are then passed towater filled lock hopper 42 wherein the ash is quenched by waterentering lock hopper 42 through line 47. The ash then settles as a layer43 near the bottom of the hopper and is periodically discharged byclosing valve 41, thereby isolating the lock hopper from the gasifier,and, upon the opening of valve 45, the water ash mixture is discharged,as a slurry, through conduit 44.

The gases produced in fluid bed 32, in admixture with the gases frompretreatment zone 8, comprise a mixture of fuel gas (i.e. a mixture ofhydrogen, carbon monoxide, carbon dioxide and methane), oils and tarsand solid particulate matter. This gas-solids mixture is held in theupper cracking section 38 of the gasification zone, which is maintainedat a temperature above 1,000F., preferably l,200-l ,500, for at leastseconds to thermally crack the oils and tars that are present into gasesand carbon. Cracking of these oils and tars increases the fuel value ofthe gas and avoids fouling problems in subsequent recovery sections. Inany event, the gas solids mixture is passed to cyclone 46 and the largerparticles are removed and passed via conduit 48 to fluid bed 32 forrecovery of their carbon values. The resultant gas-solids mixturecomprising fuel gas in admixture with very fine particulate matter ispassed via line 50 to second cyclone 52. In cyclone 52 raw product gasis recovered via line 60 for further art. For example, product-gasstream60 after suitable heat recovery and pressure reduction can be passed toa-Stretford'unit for removal-ofany sulfurcontained in the gas therebyproducing a relatively pollution free fuel.

Fine solids are removed from cyclone 52 ,via line 54 and passed tostorage bin 56 Periodically, these fine solids are withdrawn via line bysolids-feeder 58 and admixed with the air-steam mixture'of line 29. Theresultant mixture is then passed--to-venturi nozzle 36,

below constriction 37, wherein'athe carbon, present on these fineparticles is gasificedandthewery fine dust agglomerates with the ashiforwithdrawal from the system.' E

DESCRIPTION. OF THE PREFE RED EMBODIMENT 'Tabulated in Table 1 beloware'the product gases that can be produced according to'theprocess, of thepresent invention, as illustrated schematically in FIG. 1, whenprocessing an Ireland Mine Coal..The product gases are the gasesrecovered after'heat exchange to convert sensible heat to steam;expansionacross a turboexpander to generate electricity, andsulfurremoval by a Stretford process. a

- The operating conditions:

PROCESS CONDITIONS Pretreater Pressure, psia 300 1000 Temperature, F.800 800 Residence Time, Min. 30 30 Gasifier Pressure, psia 300 1000Temperature, F. 1900 1900 Residence Time, Min. 4060 40-60 PRocEssPERFORMANCE Gasifier Pressure, psia 300 1000 Product Gas Wet Dry Wet DryHeating Value, Btu/SCF Composition, ol. 153 164 CO 17.8 19.4 12.5 13.7C0,, 9.2 10.0 13.6 14.8 H 12.1 13.3 11.6 12.6 H O 8.5 8.5 CH 4.3 4.7 7.17.8 N, 48.1 52.6 46.7 51.1 Total 100.0 100.0 100.0 100.0

Thermal Efficiency, I

To all products 865 88.2 To gas only 73.2 73.9 To steam only 12.7 12.6

l. A process for the conversion of a finely divided carbonaceousmaterial containing ash to a fuel gas hav-' ing a heating value ofbetween 100 300 BTU per cubic foot which comprises:

i. pretreating a finely divided carbonaceous feed material by mildoxidation at a temperature of about 700800F for about 10 to minutes todestroy any caking properties of the carbonaceous feed material in a'fluidized pretreatment zone;

ii. said fluidized pretreatment zone being in direct fluid communicationwithan adjacent gasification zone; 1

iii. withdrawing heat from the pretreatment zone by indirect heatexchange with water to produce steam and to maintain said pretreatmenttemperature;

iv. passing the pretreated carbonaceous material and the resultant gasesfrom the upper portion of the pretreatment zone to said gasificationzone and passing at least a portion of the steam produced in saidpretreatment zone to said gasification zone;

v. said gasification zone being maintained at the same pressure as thepretreatment zone;

vi. maintaining the pretreated carbonaceous material as a fluidized bedin-the lower portion of the gasification zone at conditions sufficientto convert said pretreated material to ash and a gaseous mixture of fuelgas and oils and tars;

vii. passing the gaseous mixture to the upper portion of saidgasification zone above the fluidized bed and maintaining the gaseousmixture in the upper portion of the gasification zone at a temperatureof about 1,200- l,500F for about 10 100 seconds to crack the oils andtars in the gaseous mixture;

and viii. withdrawing ash from the bottom of said gasification zone.

2. A process as in claim 1 wherein said ash is agglomerated in thebottom portion of the gasification zone at a temperature above thetemperature of the fluidized bed.

3. A process as in claim 2 wherein said gaseous mixture includes fineand coarse particulate material and said mixture is separated to providea fine particulate fraction and a coarse particulate fraction, saidcoarse particulate fraction being passed to said fluidized bed in thegasification zone and said fine particulate fraction passed to thebottom portion of the gasification zone.

4. A process as in claim 1 wherein said finely divided carbonaceousmaterial includes higher density inorganic material, said higher densitymaterial being withdrawn from the lower portion of said fluidizedpretreatment zone. r r

5. A process as in claim 1 wherein the fluidized bed in saidgasification zone is maintained 'at a temperature of greater than l500Fand below the agglomeration temperature of the ash.

6. A process as in claim 1 wherein said carbonaceous material isbituminous coal.

1. A PROCESS FOR THE CONVERSION OF A FINELY DIVIDED CARBONACEOUSMATERIAL CONTAINING ASH TO A FUEL GAS HAVING A HEATING VALUE OF BETWEEN100 - 300 BTU PER CUBIC FOOT WHICH COMPRISES: I. PRETREATING A FINELYDIVIDED CARBONACEOUS FEED MATERIAL BY MILD OXIDATION AT A TEMPERATURE OFABOUT 700:-800:F FOR ABOUT 10 TO 30 MINUTES TO DESTROY AND CAKINGPROPERTIES OF THE CARBONACEOUS FEED MATERIAL IN FLUIDIZED PRETREATMENTZONE; II. SAID FLUIDIZED PREPTREATMENT ZONE BEING IN DIRECT FLUIDCOMMUNICATION WITH AN ADJACENT GASIFICATION ZONE; III. WITHDRAWING HEATFROM THE PRETREATMENT ZONE BY INDIRECT HEAT EXCHANGE WITH WATER TOPRODUCE STEAM AND TO MAINTAIN SAID PRETREATMENT TEMPERATURE; IV. PASSINGTHE PRETREATED CARBONACEOUS MATERIAL AND THE RESULTANT GASES FROM THEUPPER PORTION F THE PRETREATMENT ZONE TO SAID GASIFICATION ZONE ANDPASSING AT LEAST A PORTION OF THE STEAM PRODUCED IN SAID PRETREATMENTZONE TO SAID GASIFICIATION ZONE; V. SAID GASIFICATION ZONE BEINGMAINTAINED AT THE SAME PRESSURE AS THE PRETREATMENT ZONE; VI.MAINTAINING THE PRETREATED CARBONACEOUS MATERIAL AS A FLUIDIZED BED INTHE LOWER PORTION OF THE GASIFICATION ZONE AT CONDITIONS SUFFICIENT TOCONVERT SAID PRETREATED MATE-
 2. A process as in claim 1 wherein saidash is agglomerated in the bottom portion of the gasification zone at atemperature above the temperature of the fluidized bed.
 3. A process asin claim 2 wherein said gaseous mixture includes fine and coarseparticulate material and said mixture is separated to provide a fineparticulate fraction and a coarse particulate fraction, said coarseparticulate fraction being passed to said fluidized bed in thegasification zone and said fine particulate fraction passed to thebottom portion of the gasification zone.
 4. A process as in claim 1wherein said finely divided carBonaceous material includes higherdensity inorganic material, said higher density material being withdrawnfrom the lower portion of said fluidized pretreatment zone.
 5. A processas in claim 1 wherein the fluidized bed in said gasification zone ismaintained at a temperature of greater than 1500*F and below theagglomeration temperature of the ash.
 6. A process as in claim 1 whereinsaid carbonaceous material is bituminous coal.